Many system architectures benefit from the ability to disable an amplifier when not in use. In addition to the savings in power, if the disabled amplifier can display a high output impedance, several amplifier outputs can be bussed together to provide more complex functionality. In one particular application, a video cross-point switch, arrays are built from smaller building blocks. In order to implement this array, the crosspoint building block outputs must be able to disable to a high impedance. Without this ability, impedance matching would not be possible, and power dissipation would be greatly increased. These amplifiers are often used to provide gain. A non-inverting gain of two is common, to compensate for the loss of one half the signal amplitude when driving a back-terminated load. For gains that are non-inverting, and not unity, a resistor divider is typically used to provide scaled feedback around the amplifier. This resistor divider must be connected between the output and a voltage reference, often ground. When the output amplifier is disabled (its output is made high-impedance), the feedback network still loads the output of the amplifier. Typically, the impedance of the feedback network is much less than that of the disabled amplifier's output. This is an unacceptable load on the output bus. In a non-inverting, non-unity gain configuration, the feedback network must be isolated from the output node in order to provide low disabled output impedance.
Several methods of isolating the feedback network have been investigated. In one approach an extra unity gain buffer stage is used to buffer the feedback network from the output. The buffer stage may be an operational amplifier connected as a voltage follower, but any gain of +1 stage is acceptable. The buffer stage is cascaded with the output of the main amplifier. This buffer stage must be capable of being disabled and it must be a very high quality buffer because it is directly in the signal path. This method allows a high-performance solution but at the expense of high area and power. In another application an additional disableable buffer is used to bootstrap the feedback network only when the channel is disabled. The buffer is often an operational amplifier configured as a voltage follower, but any unity gain stage is acceptable. The buffer mirrors the output voltage back to the feedback node (i.e. inverting input of the amplifier). In normal operation, this buffer is disabled. When the amplifier is disabled, this buffer is enabled. By maintaining zero volts across the feedback resistor connected to the output, the current into the output is zero for all externally applied output voltages. This results in a very large output impedance limited by the accuracy of the buffer so this buffer should be as accurate as possible.
In yet another approach a buffer is used in series with the feedback network near the output node. When enabled the buffer drives the feedback network with a replica of the output voltage, maintaining the closed-loop gain control. This buffer may be disabled when the main amplifier is disabled. It always isolates the feedback network from the output bus whether or not the channel is enabled. The buffer is often an operational amplifier configured as a unity gain amplifier, but can be any voltage follower stage. Again this buffer amplifier must be very accurate because any errors it introduces to the feedback loop will be reflected back to the main amplifier. In addition its presence in the feedback path makes the feedback loop more difficult to stabilize. Another solution is to isolate the feedback network from ground or another reference using a series switching element. In practice, a saturated bipolar transistor is often used for this case. But these transistors have large non-linear offset voltages and large switching times. The use of an FET gives somewhat better results but it has a non-linear on-resistance and is not available in bipolar fabrication processes.